Measurement of surface temperatures of an object using the object's radiated long wave infrared spectrum known in the art.
Physical equations describing the basic process include a Radiated Emittance equation, and a Spectral Radiant Emittance equation.
The equation for Radiated Emittance, in Watts per square centimeter is as follows: EQU W=.epsilon..sigma.(T.sub.s.sup.4 -T.sub.b.sup.4)
where
.epsilon. is Emissivity (efficiency of radiation); PA1 .sigma. is Boltzman's constant; PA1 T.sub.s is the Temperature of the object in Kelvin; and PA1 T.sub.b is the Temperature of the background. PA1 .lambda. is Wavelength in microns; PA1 C1 and C2 are Radiation Constants; and PA1 T is Temperature of the heat source/target in Kelvin. PA1 a first heater element supported by the circuit board, proximate one of the ends of the heat sink, and operative to heat the heat sink; a second heater element supported by the circuit board, proximate another of the ends of the heat sink, and configured to heat the heat sink; and a fan supported by the circuit board and configured to direct gas into the housing, and out of the housing through the opening.
The equation for Spectral Radiant Emittance, in watts per unit area at a particular wavelength, is as follows: EQU W.sub.80 =C.sub.1)/(.lambda..sup.5 (e.sup.C2/.lambda.T -1))
where
The surface temperature of a target object is therefore proportional to the fourth power of its temperature and inversely proportional to its wavelength. A suitable temperature sensing device should be the most sensitive to the wavelengths including the expected temperature of the target. The temperature sensing device ideally will provide either a voltage or current that is proportional to the radiated energy W.
When selecting a temperature sensor for use in a product, the normal range of temperatures to which the temperature sensor will operate is often known. The inventor has sought to design a temperature sensor which will operate in with a target having a temperature in the range of approximately 10 to 140 degrees Celsius.
Using the spectral radiant Emittance equation, the inventor has determined that the majority of the energy for this temperature range lies in the wavelengths between 3 and 20 microns. A graph of the radiant energy distribution is shown in FIG. 1.
There are a number of types of infrared sensing devices. These include pyroelectric detectors, cryogenic photovoltaic sensors, semiconductor junctions and infrared thermocouples. While pyroelectric devices are very sensitive in this temperature range, they are transient detectors, requiring an expensive mechanical chopper to look at a constant temperature target. They also have a very slow response time. Cryogenic devices are fast and very sensitive but require very expensive coolers to reach their 77-100 Kelvin operating ranges. Semiconductor devices are fast and inexpensive, but have a very narrow range of sensitivity around 0.8 microns, outside the desired range.